Optical phase modulation has been carried out on plane waves of the Gaussian type, i.e. having a field intensity which is greater in the middle than near the periphery, by using the optical Kerr effect. The Kerr effect relates to the property of some materials which produce self-induced variations of refractive index when waves of different powers pass therethrough. Such non-linear propagation materials can thus be used to set up phase shifts in the plane wave as it penetrates the non-linear material.
However, when the power of the plane wave is too small, the phase shift remains less than 2.pi. radians and is therefore too small to be applied advantageously. In contrast, when the power of the plane wave is high, self-focussing occurs within the non-linear material. This produces to an unstable and uncontrollable phase modulation phenomenon which causes the wave to explode within the material.
There is thus a problem of being able to create stable and controllable phase modulation in a light beam of sufficient magnitude without causing the light beam to explode within the non-linear material.
It was proposed, in particular in an article by M. Vampouille and J. Marty entitled "Controlled Phase Modulation in Monomode Optical Fibers" published in the journal "Optical and Quantum Electronics" No. 13, 1981 pp. 393-400, to provide controlled phase modulation by using non-Gaussian pulses which are shaped as desired by using suitable time relationships and which are transmitted along monomode and non-dispersing optical fibers.
The use of monomode optical fibers to create controlled phase modulation nevertheless represents a major constraint which greatly limits the range of applications. Monomode fibers have a very small diameter core and can therefore handle minute quantities of power without suffering damage. In some ranges of wavelength, it is no longer possible, in practice, to work only with fibers made of silica, i.e. a monomode material which is only slightly non-linear and which does not enable really interesting performance to be obtained.
The phenomenon of self-focussing of a laser beam comprising pulses whose initial durations are about 30 ps when the laser beam penetrates into a material which exhibits the optical Kerr effect has been analyzed in an article by M. Vampouille, B. Colombeau and C. Froehly entitled "The Application of Self-Focussing Control in CS.sub.2 to shortening Picosecond Laser Pulses" which appeared in the journal "Optical and Quantum Electronics" No. 14, 1982, pp 253-261. Although this article provides experimental conditions for attaining the spatial distribution of the self-focussing phenomenon in non-linear optical materials and describes the characteristics of the modulation effects obtained, it fails to indicate how to avoid self-focussing of the incident plane waves which are always of the Gaussian type. The range of applications of the devices describes in this article are therefore limited. In particular, how a stable beam may be created which propagates in a perfectly controlled manner with non-linear phase shifts of greater than 2.pi. radians is not taught by the prior art.
It is an object of the present invention to provide controlled and stable phase modulation of a beam having a regular geometrical structure by enabling phase shifts of greater than 2.pi. radians to be obtained.
Another object of the invention is to provide high gain amplification of beams while maintaining a regular geometrical structure and avoiding critical self-focussing problems.